Raw material for shot-peening materials, finished wire, method of manufacturing shot-peening materials, and shot-peening materials

ABSTRACT

The purposes of the present inventions are to provide a raw material for shot-peening materials wherein breaking a wire is prevented in obtaining a finished wire to improve the productivity, to provide a finished wire and a method of manufacturing shot-peening materials by which productivity is improved, and to provide shot-peening materials that are manufactured by that method. The finished wire, of which the area of carbides with a particle size of 2 μm or less is 80% or more of the total area, is manufactured by the steps of wiredrawing a raw material to obtain a wire, and repeatedly annealing and cold-drawing the wire. The raw material for the shot-peening materials comprises, by mass %, 0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less manganese, 0.020% or less phosphorus, 0.010% or less sulfur, 1.40-1.60% chromium, 0.0015% or less oxygen, and the remaining materials of iron and unavoidable impurities. The method of manufacturing the shot-peening materials uses that raw material. The shot-peening materials are manufactured by that method.

TECHNICAL FIELD

The present inventions relate to a raw material for shot-peeningmaterials, a finished wire, a method of manufacturing shot-peeningmaterials, and shot-peening materials.

BACKGROUND ART

In an existing process of manufacturing shot-peening materials, a rawmaterial, e.g., so-called bearing steel, is wiredrawn to obtain a wire.The wire is cut at the same length as its diameter. The cut wire is thenthrown at a rigid wall to round off its edges. The rounded-off wire isground to a predetermined degree of sphericity. Then, it is quenched andtempered to get a predetermined Vickers hardness. This process is wellknown (e.g., Japanese Laid-open Publication No. 2001-79766).

DISCLOSURE OF INVENTION

However, when a bearing steel that is not modified from a standard steel(e.g., a high carbon chromium bearing steel, SUJ2 of JIS (JapanIndustrial Standards) G4805, of which the components are shown in Table2) has been used as a raw material for shot-peening materials, there hasbeen a problem of the wire breaking. This does not improve theproductivity of the shot-peening materials. That is, once a wire breaksin a process for wiredrawing, a line for manufacturing a wire, whichruns at 100 m/min, stops. Thus, the productivity is significantlyaffected. As a result, the productivity of shot-peening materials cannotincrease. Especially, for a wire having a diameter of less than 0.7 mm(for example, a diameter of 0.3 mm), a wire often breaks in awiredrawing process and the problem becomes significant. Thus, the mostappropriate method of manufacturing shot-peening materials from a smalldiameter finished wire has not been established.

The purpose of the present inventions is to solve that problem and toprovide a raw material for shot-peening materials that is used forpreparing a finished wire by wiredrawing. Breaking a wire is thusprevented in wiredrawing, so as to improve the productivity of theshot-peening materials. Another purpose is to provide a finished wire.Other purposes are to provide a method of manufacturing shot-peeningmaterials by which improved productivity is achieved and to provideshot-peening materials that are manufactured by that method. That is, inthe method a specific raw material for shot-peening materials is usedfor maintaining its purity to prevent a wire from breaking. A finishedwire that is appropriate for subsequent processes is obtained from theraw material. The good structure of the raw material is maintaninedafter quenching and tempering by retaining fine carbides in thestructure before quenching. The present inventions also provideshot-peening materials that have long lives and that induce appropriatecompressive residual stress in a work. The present inventions furtherprovide a method of manufacturing shot-peening materials that arereadily quenched, to be suitable for shot-peening.

As the result of the inventors' keen study to solve the problem, theyhave discovered that a raw material for shot-peening materials thatcontains by percentages by mass 0.95-1.10% carbon, 0.15-0.30% silicon,0.40% or less manganese, 0.020% or less phosphorus, 0.010% or lesssulfur, 1.40-1.60% chromium, 0.0015% or less oxygen, and the remainingmaterials of iron and unavoidable impurities, can achieve the purpose.The first feature of the present inventions has been made based on thatfinding. This raw material is selected to reduce non-metallicinclusions, which deteriorate the connections in a base metal, toprevent a wire from breaking by cracking. That is, there is a need toreduce oxides, compared to the method of manufacturing shot-peeningmaterials disclosed in Japanese Patent Laid-open No. 2001-79766. Thus,the content of oxygen is significantly reduced to 0.0015% or less. Themaximum content of silicon is reduced by 17%. The contents of manganeseand phosphorus are also reduced to reduce sulfides, especially MnS. Ingeneral the maximum contents other than iron are reduced. The content ofcarbon is unchanged, to maintain the amount of carbides, which areessential to the material.

The finished wire for the shot-peening materials of the presentinventions is manufactured by the steps of wiredrawing the raw materialof the first feature of the present inventions to obtain a wire, andrepeatedly annealing and cold-drawing the wire to obtain a finishedwire. The area of carbides with sizes of 2 μm or less is 80% or more ofthe total area of the finished wire. That is, a wire is manufactured by“wiredrawing” an ingot. Next, the wire is repeatedly “annealed” and“cold-drawn” to become a finished wire. Then, the carbides of thefinished wire are at a condition where the area of carbides with sizesof 2 μm or less is 80% or more of the total area of the finished wire.In the second feature of the inventions, the wire that is obtained bywiredrawing the raw material for the shot-peening materials of the firstfeature of the inventions is repeatedly annealed and drawn, because itsability to elongate decreases and its breaking becomes easier when therate of reduction of the cross-sectional area of it is high. Thus, it isannealed to eliminate work hardening when its elongation decreases suchthat the cross section is not reduced as before. It is preferable towiredraw it again under the condition where its elongation is recovered.It is cold-drawn, because the refinement of the crystalline grains andwork hardening occur under cold-drawing. By hot-drawing it, crystallinegrains are elongated, and no refinement occurs. The annealing ispreferably done three, four, or five times. The reduction of thecross-sectional area of the wire is preferably 10% to 40%. The area ofcarbides with particle sizes of 2 μm or less is 80% or more of the totalarea of the finished wire, because then a structure appropriate for theshot-peening materials is maintained in a small diameter “finishedwire.” By maintaining the structure of the finished wire to be one thatis appropriate, the quality of the final product is improved and theloss of raw materials is avoided.

The term “finished wire” is defined as a small diameter wire that isfinished before being cut. It is obtained by repeatedly annealing andcold-drawing a wire that is prepared by rolling or drawing.

The sizes of the carbides of the finished wire are maintained to be thesame as those before quenching by annealing at 720° C. or lower, becausethe coarsening of carbides is prevented by the annealing. Therefore, alarger force can be applied to a work by shot-peening materials, becausethe breaking strength of them increases as a result of the refinement ofthe crystalline grains. Annealing is preferably performed at 700° C. byusing a bright annealing furnace. The use of it eliminates an acidcleaning process, since no oxidized scales are formed on the surface ofthe wire.

The method of manufacturing the shot-peening materials of the presentinventions is characterized in that it comprises the steps of cuttingand plastic-forming the finished wire of the second feature to make rawshot-peening materials, and quenching and tempering them. The presentinventions provide shot-peening materials that are efficientlymanufactured and have a good quality, since they are made from thefinished wire, which has a proper elongation and has a reliable quality.

The tempering parameters are defined by the following equation: T{(21.3−5.8×[C])+log(t)}: where T denotes the tempering temperature (K), t thetempering time (hr), and [C] the carbon content (%). The temperingtemperature T and the carbon content C are selected so that theparameters become 6200-7300. When the tempering temperature T, thecarbon content C, and the tempering time t, are selected so that theparameters become 6200-7300, the coarsening of crystalline grains isprevented and the residual stress is relieved, to increase toughness.Thus, that selection is preferable. The method may further comprise thestep of plastic-forming the raw shot-peening materials. This method hasan advantage in that almost no breaking is generated during ashot-peening process, because the corners of the shot-peening materialsare rounded off. Plastic-forming the raw shot-peening material means tomake the raw shot-peening material in a spherical shape from a shortpiece that is made by cutting the finished wire by plastic-forming. Thatis, the raw shot-peening materials are not rounded by cutting orgrinding.

The method of manufacturing the shot-peening materials of the presentinventions is further characterized in that it comprises the steps ofwiredrawing a raw material for the shot-peening materials of the firstfeature to obtain a wire, repeatedly annealing and cold-drawing the wireto obtain a finished wire, obtaining the raw shot-peening materials bycutting the finished wire and plastic-forming, and quenching andtempering the raw shot-peening materials. It has an advantage in thatthe crystalline grains are refined and the toughness is recovered afterthe heat treatment.

The method of manufacturing the shot-peening materials of the presentinventions is further characterized in that the quenching temperature is820-850° C. in the aforementioned method. It has an advantage in thatalmost no residual austenite is generated, and overall, martensite isuniformly obtained in the structure.

The shot-peening materials of the present inventions are thosemanufactured by the methods that are described above. They substantiallyhave a structure of tempered martensite as a base metal in which finecarbides precipitate. The ratio of the area of the carbides to the totalarea is preferably 70-95%. When the metallic portion, which is a binder,becomes less, the binding function of the base metal become weaker. Whenthe metallic portion is significantly reduced such that the ratioexceeds 95%, the carbides come close to each other, resulting inweakening the binding function of the base metal. If the ratio is lessthan 70%, the shot-peening materials will not have the requiredhardness, such as a Vickers hardness of 950 HV, which is appropriate fora material used for shot-peening materials, such as JIS SUJ2. Thepresent inventions can provide materials that are suitable forshot-peening. The sizes of the carbides are preferably smaller than 2 μmin diameter, more preferably 1-0.1 μm. When they are 2 μm or larger,they often cause a crack in the shot-peening materials. When they are1-0.1 μm in diameter their effect becomes less, because their exposureon the surface of the base metal is reduced. Thus, that is preferable.The raw material of the shot-peening materials of the present inventionscomprises by percentages by mass 0.95-1.10% carbon, 0.15-0.30% silicon,0.40% or less manganese, 0.020% or less phosphorus, 0.010% or lesssulfur, 1.40-1.60% chromium, 0.0015% or less oxygen, and the remainingmaterials of iron and unavoidable impurities. The raw material iswiredrawn to be a wire. The wire is repeatedly annealed and cold-drawnto be a finished wire. The finished wire is cut and subjected toplastic-forming to be raw shot-peening materials. The shot-peeningmaterials are obtained by quenching and tempering the raw shot-peeningmaterials. Their structures consist of fine carbides and temperedmartensite. The ratio of the area of carbides to the total area is70-95%. The Vickers hardness of them is preferably adjusted to 950 HV to1050 HV by plastic-forming. When their Vickers hardness is 950 HV to1050 HV a work having high hardness can be properly processed byshot-peening.

Since the raw material of the present inventions is selected so as toreduce the possibility of breaking a wire, the productivity is improved.In addition, even a small diameter wire can be processed at a highspeed. The finished wire of the present inventions is suitable formanufacturing shot-peening materials. The method of manufacturing theshot-peening materials of the present inventions is high in bothproductivity and quality. The shot-peening materials of the presentinventions are suitable for a shot-peening process.

The basic Japanese patent applications, No. 2008-046967, filed Feb. 28,2008, and No. 2008-169971, filed Jun. 30, 2008, are hereby incorporatedin their entirety by reference in the present application. The presentinventions will become more fully understood from the detaileddescription given below. However, the detailed description and thespecific embodiment are illustrations of desired embodiments of thepresent inventions, and are described only for an explanation. Variouspossible changes and modifications will be apparent to those of ordinaryskill in the art on the basis of the detailed description. The applicanthas no intention to dedicate to the public any disclosed embodiment.Among the disclosed changes and modifications, those which may notliterally fall within the scope of the present claims constitute,therefore, a part of the present inventions in the sense of the doctrineof equivalents. The use of the articles “a,” “an,” and “the” and similarreferents in the specification and claims are to be construed to coverboth the singular and the plural, unless otherwise indicated herein orclearly contradicted by the context. The use of any and all examples, orexemplary language (e.g., “such as”) provided herein, is intended merelyto better illustrate the inventions, and so does not limit their scope,unless otherwise claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of the method of manufacturing the shot-peeningmaterials of the present inventions.

FIG. 2 a shows a microstructure of the finished wire for theshot-peening materials of the present inventions photographed by a SEM(×1,000).

FIG. 2 b shows a microstructure of the finished wire for theshot-peening materials of the present inventions photographed by a SEM(×5,000).

FIG. 3 a shows a microstructure of the shot-peening materials of thepresent inventions photographed by a SEM (×1,000).

FIG. 3 b shows a microstructure of the shot-peening materials of thepresent inventions photographed by a SEM (×3,000).

FIG. 4 a shows a microstructure of the shot-peening materials of thepresent inventions photographed by a SEM (×1,000).

FIG. 4 b shows a microstructure of the shot-peening materials of thepresent inventions photographed by a SEM (×3,000).

FIG. 5 is a graph showing the distribution of the sizes of the carbidesof the shot-peening materials of the present inventions.

FIG. 6 a is a graph showing the relationship between the ratio of thearea of carbides and the hardness (a hardness derived from valuesobtained by the hardesses of two respective structures) of theshot-peening materials of the present inventions (for a structure havinga higher hardness).

FIG. 6 b is a graph showing the relationship between the ratio of thearea of carbides and hardness (a hardness derived from values obtainedby the hardesses of two respective structures) of the shot-peeningmaterials of the present inventions (for a structure having a lowerhardness).

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors provide shot-peening materials that are suitable for ashot-peening process and that are made to form a wire, by adjusting thecomponents of a bearing steel, SUJ of JIS G4805, to the components thatare suitable for a wire for shot-peening materials and that emphasizethe features of the components. The components of the raw material ofthe shot-peening materials of the present inventions are shown inTable 1. The components of the existing material, SUJ2 (the JapaneseIndustrial Standards), are shown in Table 2 as a comparative example.

TABLE 1 (Percentages by mass) Iron and unavoid- able impu- CarbonSilicon Manganese Phosphorus Sulfur Chromium Oxygen Copper NickelMolybdenum rities Working 0.95-1.10 0.15-0.30 ≦0.4 ≦0.02 ≦0.01 1.40-1.60≦0.0015 ≦0.15 ≦0.15 ≦0.06 Remaining Example

TABLE 2 (Percentages by mass) Iron and unavoid- able impu- CarbonSilicon Manganese Phosphorus Sulfur Chromium Oxygen Copper NickelMolybdenum rities Comparative 0.95-1.10 0.15-0.35 ≦0.50 ≦0.025 ≦0.0251.30-1.60 Not spec- Not spec- Not spec- Not spec- Remaining Exampleified ified ified ified

As shown in Tables 1 and 2, the raw material of the present inventionhas 0.010% or less sulfur and 0.0015% or less oxygen, which are bothlower than those of the bearing steel, SUJ, of JIS G4805. Thus, theprecipitation of impurities such as sulfides and oxides is reduced andthe purity of the material is maintained. So, a non-uniform structure,which may break a wire, is prevented. As a result, breaking a wire isprevented or minimized. Breaking a wire during wiredrawing is preventedeven though the wire is small in diameter.

The contents of manganese and phosphorus are limited to lower values,compared to the percentage of the bearing steel, SUJ, of JIS G4805.These limitations are preferable to suppress the generation of residualaustenite and an intergranular ternary compound.

The content of copper is preferably limited to 0.15% or less to avoidthe deterioration of carburizing.

The content of nickel is preferably limited to 0.15% or less to avoiddeterioration of carburizing.

Example 1

The flowchart of manufacturing shot-peening materials by using the rawmaterial for the shot-peening materials of the present inventions isshown in FIG. 1. By reference to FIG. 1, manufacturing shot-peeningmaterials is described below. As in the flowchart, at a first step a rawmaterial having the components of Table 1 is prepared. At a second stepthe raw material is wiredrawn to be a wire. At a third step the wire isrepeatedly annealed and cold-drawn. At a fourth step the wire is cut. Ata fifth step the cut wire is subject to plastic formation. At a sixthstep it is quenched and tempered. At a seventh step it is shot just toharden the shot-peening materials themselves.

Comparisons between example 1 and the bearing steel of Table 2, SUJ ofJIS G4805, during the first three steps, are shown in Table 3. The wireafter the wiredrawing of the second step has a diameter of 1.6 mm and ahardness of 320 HV.

TABLE 3 Working Example Comparative Example Containing 0.95-1.10%carbon, Containing 0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less0.15-0.35% silicon, 0.50% or less manganese, 0.020% or less manganese,0.025% or less phosphorus, 0.010% or less sulfur, phosphorus, 0.025% orless sulfur, 1.40-1.60% chromium, 0.0015% or 1.30-1.60% chromium, andthe less oxygen, 0.15% or less copper, remaining materials of iron and0.15% or less nickel, 0.06% or less unavoidable impurities molybdenum,and the remaining materials of iron and unavoidable impurities Breakinga wire did not occur at the Breaking a wire occurred at the step ofwiredrawing to obtain a step of wiredrawing to obtain a finished wire inmanufacturing the finished wire in manufacturing shot-peening materials.the shot-peening materials.

As shown in FIG. 3, no breaking of a wire occurred at the step ofwiredrawing to obtain a finished wire in manufacturing the shot-peeningmaterials in Example 1. By contrast, breaking a wire occurred at thatstep in the comparative example.

TABLE 4 Working Example Comparative Example Containing 0.010% or lesssulfur Containing 0.025% or less sulfur and 0.0015% or less oxygenBreaking a wire did not occur at the Breaking a wire occurred at thestep of wiredrawing to obtain a step of wiredrawing to obtain a finishedwire in manufacturing the finished wire in manufacturing shot-peeningmaterials. the shot-peening materials.

The finished wire for the shot-peening materials is manufactured byrepeatedly annealing and cold-drawing at the third step. The diameter ofthe wire for the finished wire is 1.6 mm and its hardness is 320 HV.Since the wire is not work-hardened and has the hardness of 320 HV,cold-drawing can be easily performed. Specifically, at the third step,where annealing and cold-drawing are repeated, a bright annealingfurnace (BA) that is 8 m long is used to heat one portion, which is 4 mlong, at 700° C. and to cool the other portion, which is 4 m long. Anexample of the changes of the sizes, which example includes annealingfour times, is as follows:

Diameter: 1.6 mm-1.5 mm-1.4 mm→(Annealing in the BA)→1.3 mm-1.2 mm-1.1mm-1.0 mm-→(Annealing in the BA)→0.9 mm-0.8 mm-0.75 mm-0.7 mm (Annealingin the BA)→0.6 mm-0.55 mm-0.5 mm-0.45 mm-0.4 mm→(Annealing in theBA)→0.35 mm-0.3 mm (End).

TABLE 5 Comparative Example 1 Working Example Comparative Example 2Repeatedly annealing and cold- Repeatedly annealing and cold- Repeatedlyannealing and cold- drawing to obtain a finished wire drawing to obtaina finished wire drawing to obtain a finished wire with a diameter ofmore than 0.6 mm with a diameter of 0.25-0.6 mm with a diameter lessthan 0.25 mm The annealing is performed at a The annealing is performedat a temperature of 720° C. or less. temperature over 720° C. When thediameter exceeds 1.0 mm, Breaking a wire occurs at the Cutting a wirewith a diameter the wire is too thick for shot- step of wiredrawing toobtain of less than 0.25 mm is difficult. peening materials. thefinished wire in manufacturing When the annealing temperature theshot-peening materials. exceeds 720° C., the It is necessary thatwiredrawing temperature is over the be properly performed bytransformation temperature, annealing, to relieve the residual and sothe possibility increases stress and soften the steel. that thestructure of a base metal will change.

The observation of the structure of the finished wire shows that thearea of carbides with particle sizes of 2 μm or less is 80% or more ofthe total area of the finished wire. A microstructure of the finishedwire (with a diameter of 0.3 mm and after being etched by nital) for theshot-peening materials of the present inventions photographed by a SEMis shown in FIG. 2. It shows that the structure of the finished wirethat is good for shot-peening materials is obtained. The diameter of thefinished wire is arranged to be 0.25-0.6 mm by repeatedly annealing andcold-drawing. When the final diameter is 0.3 mm, the Vickers hardness is350 HV. Even though the Vickers hardness is 350 HV, cutting the smalldiameter wire is easy because the material has not been greatlywork-hardened. In Example 1, manufacturing the shot-peening materialsthat have a diameter of 0.3-0.6 mm is described. However, shot-peeningmaterials that have a diameter up to about 1.0 mm are usable. Theshot-peening materials that have a diameter of 1.0 mm can be easilymanufactured by a method other than that of the present inventions. Thesmaller the diameter of the shot-peening materials is, the moredifficult manufacturing them is. From this point of view, the method ofthe present inventions is appropriate for manufacturing the shot-peeningmaterials that have a diameter of 0.6 mm or less (the finished wire thathas a diameter of 0.6 mm or less). The diameter of shot-peeningmaterials can be measured, for example, by the method of measuring aparticle under the Japanese Industrial Standards as specified by JISG5904.

TABLE 6 Working Example Comparative Example 1 Comparative Example 2 Thearea of carbides The area of carbides The area of carbides with particlesizes with particle sizes with particle sizes of 2 μm or less is of 2 μmor less is of more than 2 μm is 80% or more of the less than 80% of the80% or more of the total area. total area. total area Cutting the wireCutting the wire is Cutting the wire is is easy. difficult because it isdifficult and the wire considerably work- is easy to break. hardened,even though its hardness is 350HV.

At the fourth step the finished wire is cut. One end of it is abuttedagainst a stopper. It is fixed by a wire-gripping mechanism forpreventing buckling and maintaining a constant length withoutfluctuation. Then it is cold-sheared by a cutting die. A cold-cuttingmachine with a mechanical press driven by a cam of a crank shaft, or apress driven by fluid pressure, or an electrical press, is used. Adieing machine may also be used. The cut length is arranged to be fromthe same length to one and half times the diameter of the finished wire.

At the fifth step, plastic-forming is applied to the cut wire. Forexample, it is rounded by press-forming. Alternatively, it is shotagainst a wall at a high speed to round off the corners.

At the sixth step it is quenched and tempered. In Example 1 thestructure and hardness are adapted to suit quenching and tempering. Thetempering parameters, which are shown in Table 7, are adjusted to be6200-7300. The effects are shown in Table 7.

TABLE 7 Comparative Example Working Example Comparative ExampleTempering parameters <6200 Tempering parameters = Temperingparameters >7300 T((21.3 − 5.8 × [C %]) + log[t]): 6200-7300 The life ofshot-peening Their toughness and Residual compressive materials is shortbecause hardness are satisfactory. stress is not provided to they lacktoughness. Their lives are extended. a work because its Residualcompressive hardness is low. stress is provided to a work.

As shown in Table 7, the life of the shot-peening materials is improvedby adjusting the tempering parameters. The non-uniform structure causedby the precipitation of impurities has an impact on the life. Thefinished wire in which the area of carbides with sizes of 2 μm or lessis 80% or more of the total area is used in the embodiment (see FIG. 2).This has a good impact on the life. In addition, an appropriatecondition of shot-peening (such as making the hardness of theshot-peening materials the same as, or more than, that of a work) has agood impact on the life, as described below. The Vickers hardness of theshot-peening materials that have a diameter of 0.6 mm is 940 HV. That ofthe materials in the embodiment that have a diameter of 0.3 mm is 960HV. FIG. 2 a shows a microstructure of the finished wire that has adiameter of 0.3 mm, as photographed by a SEM (×1,000). FIG. 2 b shows amicrostructure of the finished wire that has a diameter of 0.3 mm, asphotographed by a SEM (×5,000).

Observations of the structures of the shot-peening materials of theworking example and the comparative example are described below. Thestructure of the materials of the working example is composed of finecarbides and tempered martensite (see Table 8). This structure has aneffect in that the materials are seldom broken by a break beinginitiated at the area of an impurity. This effect is apparent when thestructures of the materials before and after shot-peening are compared.The microstructures of the shot-peening materials that have a diameterof 0.6 mm (before shot-peening) and a diameter of 0.3 mm (aftershot-peening) as photographed by a SEM after being etched by nital areshown in FIGS. 3 and 4. FIG. 3 a shows a microstructure of theshot-peening materials that have a diameter of 0.6 mm beforeshot-peening. It is photographed by a SEM (×1,000). FIG. 3 b shows amicrostructure of the shot-peening materials that have a diameter of 0.6mm before shot-peening. It is photographed by a SEM (×3,000). FIG. 4 ashows a microstructure of the shot-peening materials that have adiameter of 0.3 mm after shot-peening. It is photographed by a SEM(×1,000). FIG. 4 b shows a microstructure of the shot-peening materialsthat have a diameter of 0.3 mm after shot-peening. It is photographed bya SEM (×3,000).

TABLE 8 Working Example Comparative Example The structure is composed offine The structure is composed of fine carbides carbides with sizes ofless than 2 μm with sizes of 2 μm or larger and and tempered martensite.imperfectly tempered martensite. The materials are seldom broken by Whena broken shot-peening material is a break being initiated at the area ofmixed into the shot-peening materials, it an impurity duringshot-peening. possibly causes a scar on a work and initiates a break.

TABLE 9 Comparative Example 1 Working Example Comparative Example 2 Thequenching The quenching The quenching temperature is lower temperatureis temperature is higher than 820° C. 820-850° C. than 850° C. Thesolution of carbides is The solution of The solution of too little inquantity to get carbides is carbides is too fine carbides. appropriate.great in quantity.

The shot-peening materials that are obtained in Example 1 and thecomparative examples are described below (see Table 9). As shown inTable 9, the carbides are appropriately dissolved when the quenchingtemperature is 820-850° C. FIG. 5 shows a graph of the distribution ofthe sizes of the carbides of the shot-peening materials of FIG. 3 b.Each carbide is projected on a grid paper to measure its area. The sizeof each carbide is calculated as the square root of the area. Thus, thesizes of the carbides and their distribution are obtained. The averagesize is 0.8 μm. The sizes range between 0.5 and 2.0 μm. Since thecarbides of less than 0.5 μm are substantially unmeasurable, they areexcluded from the measurement. The average size is calculated as anumeric mean.

Example 2

The conditions and results of shot-peening in which the shot-peeningmaterials of the present inventions are shot against a work are shown inTable 10. Example 2 shows that the shot-peening materials of the presentinventions, of which the hardness is 950 HV, cause a smaller amount tobe scraped off when they are shot against a hard work. They also causework-hardening and larger residual stress in the work (see ExperimentNos. 11-17). The hardness of the materials is initially 950 HV, butpossibly increases to 1050 HV after shot-peening.

The amount being scraped off is measured as follows:

A Method of Measuring an Amount Being Scraped Off

The diameter of a work before shot-peening and that after shot-peeningare measured by a laser dimension-measuring device. The amount iscalculated by the following equation. It is the mean value of tenmeasurements (n=10). The measurement is performed at the center of thetarget of the shot (where the maximum amount being scraped off would bemeasured). The amount being scraped off=(D1−D2)/2, where D1 denotes thediameter of the work before shot-peening, and D2 the diameter of thework after shot-peening.

The residual stress of a work is measured as follows:

A Method of Measuring a Residual Compressive Stress

The method of measuring the residual compressive stress of a work aftershot-peening is an X-ray stress measuring method. It is commonly used asa non-destructive measurement, and provided in JIS B 2711. The methodutilizes the diffraction of X-rays. Since the specimen of themeasurement is a steel having a martensite structure, the measurement isperformed using CrKα characteristic radiation and an X-ray stressconstant of k=−318)(MPa/°. The measurement is performed at the center ofthe target of the shot. Before the measurement, an area that isapproximately double the cross-sectional dimension of the incident X-raybeam is removed from the specimen by electrolytic grinding. The peakvalue of the residual compressive stress (the maximum value) is obtainedbased on the distribution of the measurement of the residual stress.

The hardness of the cross section of a work is measured as follows:

A Method of Measuring the Hardness of the Cross Section

HV0.3 in Table 10 means a Vickers hardness of a cross section measuredat a depth of 50 μm from the surface with a load of 300 g. It isgenerally known that the hardness is very low near the surface ofgas-carburized steel, because an oxidized and nonmartensitic layer isformed from the surface to a depth of about 25 μm. Thus, the measurementof such a portion is not useful for the evaluation of the material andthe heat treatment. So, the measurement is performed on the crosssection.

A Method of Measuring the Relative Hardness

The relative hardness in Table 10 means a value that is calculated bysubtracting the hardness of the shot-peening materials from that of thesurface of the work. The hardness of the surface of the work is measuredat its surface. While the hardness at the cross section is used forevaluating the material and heat treatment, the hardness at the surfaceis important for selecting the shot materials. Thus, the hardness ismeasured by putting an indenter directly on the surface. The hardness ismeasured by a micro-Vickers hardness tester (a load of 500 g).Therefore, for a carburized steel the measured hardness includes a valueof an oxidized and nonmartensitic layer. Though an oxidized andnonmartensitic layer is not formed in a vacuum-carburized steel, thehardness of a surface would decrease, depending on the characteristicsof the quenching.

The ratio of the area of carbides is preferably 70-95%, more preferably80-95%. FIGS. 6 a and 6 b are graphs showing the relationships betweenthe ratios of the area of carbides and the hardness (a hardness derivedfrom values obtained by the hardesses of two respective structures) ofthe shot-peening materials of the present inventions (for structureshaving a higher hardness and a lower hardness, respectively). Thesegraphs illustrate that the Vickers hardness is 920 HV-1030 HV when theratio of the area of carbides is 70-95%. For the hardness of 950 HV theratio is 70-78%. The hardness HV(m) of the shot-peening materials can becalculated by the following equations, 1, 2, and 3.

HV(m)={f(C)−f(T,t)}(1−γ_(R)/100)+400×γ_(R)/100  (1)

f(C)=−660C ²+1373C+278  (2)

f(T,t)=0.05T(log(t)+17)−318=0.052λ−318  (3)

where

-   -   C: Carbon content (mass %) at the surface after carburizing,    -   T: Tempering temperature (K),    -   t: Holding time for tempering (hr),    -   γ_(R): Amount of residual austenite (volume %),    -   λ: Tempering parameters (6200-7300),    -   f(C): Maximum hardness of martensite (HV), and    -   f(T,t): Hardness decreased by tempering (HV).

The following equation is obtained by substituting equations 2 and 3 forequation 1.

HV(m)=(−660C ²+1373C+596−0.05λ)(1−γ_(R)/100)+4γ_(R)  (4)

The carbon content is assumed to be 0.75%. This value is used as theallowable limit of the carbon content in the matrix, i.e., martensite.The carbides, which precipitate, are excluded by assuming that theircarbon content is 0.75%. Thus, the equation is used for calculating thehardness of the matrix. The residual austenite γ_(R) that remains afterthe heat treatment is estimated as follows:

Ms=667−195C−44.9Mn−19.6Ni−21.4Cr−20.7Mo

γ_(R)=100·exp(−0.011(Ms−T _(q))),

where M_(s) denotes the temperature of the initiation of martensitetransformation, and T_(q) the lowest temperature achievable duringquenching. The hardness derived from values obtained by the hardesses oftwo respective structures of the shot-peening materials, which arecalculated by these equations, is shown in FIGS. 6 a and 6 b.

In comparative example 1, the shot-peening materials (CCW) (having thehardness of 700 HV and the compositions including, by mass %, 0.81%carbon, 0.48% manganese, 0.23% silicon, 0.012% phosphorus, 0.004%sulfur, and unavoidable impurities) are made by cutting a steel wire androunding off the cut wire. They do not induce a high residual stress ina work having a high hardness, such as super carburizing steel. (It is asteel appropriate for carburizing by dispersing carbides, and has aVickers hardness of 880 HV-990 HV.) (See Experiment No. 8.) Thecorresponding example of the present inventions is Experiment No. 15. Incomparative example 2, the super-hard shot-peening materials (having aVickers hardness of 1380 HV) are too hard, and so have a disadvantage inthat a work (JIS SCM420H, a quenched steel) is scraped off too much.Thus, a shot-peening pressure is controlled to be low when they are usedas the shot-peening materials (see Experiment No. 10). The correspondingexample of the present inventions is Experiment No. 11.

TABLE 10 Experiment Heat Shot-Peening Shot-Peening No. Work TreatmentMaterials Conditions 1 SCM420H Gas Eutectoid Comparison Condition 1:Carburizing Example 1 CCW (700HV, 0.6 mm-dia.), 2 Vacuum Shot Pressure0.3 MPa, 3 Eutectoid Coverage 300%, 4 Super Carburizing Arc-Height 0.498mmA 5 Carburized  5 6 Steel 10 7 20 8 30 9   20S 10 SCM420H VacuumComparison Condition 2: Super-Hard (HV1380, Eutectoid Example 2 0.2mm-dia.), Shot Pressure 0.5 MPa, Carburizing Coverage 300%, Arc-Height0.2 mmA 11 Present Condition 3: 12 Super Invenitions Present Inventions13 Carburized  5 (HV950, 0.3 mm-dia.), 14 Steel 20 Shot Pressure 0.5Mpa, 15 30 Coverage 300%, 16   20S Arc-Height 0.372 mmA Residual PeakStress at Residual Position of Experiment Measurement of SurfaceRelative Surface Stress Peak Residual No. Scraping off HV0.3 Hardness(MPa) (MPa) Stress (μm) 1 0.0 524 175.6 −431 −1323 80 2 0.0 734 69.5−521 −1386 60 3 0.0 770 −9.8 −538 −1271 80 4 0.0 897 −64.7 −514 −1362 805 0.0 923 −92.4 −356 −1215 100 6 0.0 950 −9.2 −578 −1108 60 7 0.0 988−172.6 −517 −1178 100 8 1.0 963 −213.8 −422 −1180 80 9 0.6 958 −255.5−639 −1123 60 10 81.7 1178 437.9 −1269 −2088 30 11 0.0 1077 11.2 −1167−2041 50 12 0.0 918 244 −1100 −1724 50 13 0.0 998 144.8 −951 −1625 60 140.0 1126 108 −932 −1590 60 15 0.0 1045 10.5 −675 −1532 70 16 0.0 1106−1.3 −929 −1673 60 Note 1: A number in the “Heat Treatment” columnindicates the ratio of area of carbides that precipitate insuper-carburized steel. Note 2: A number with a suffix “s” in the “HeatTreatment” column (20S) indicates that the material is prepared by asubzero treatment at the ratio of area of carbides of 20%.

As described above, the present inventions provide a raw material forshot-peening materials that is used for preparing a finished wire bywiredrawing wherein breaking a wire is prevented in manufacturing theshot-peening materials. Thereby the productivity is improved. Thepresent inventions also provide the finished wire, a method ofmanufacturing the shot-peening materials by which the productivity isimproved, and shot-peening materials that are manufactured by thatmethod. They provide shot-peening materials that have a long life andinduce appropriate residual stress in a work. In addition, they providea method of manufacturing shot-peening materials that are readilyquenched, and thereby appropriate for shot-peening.

1. A raw material for shot-peening materials comprising, by percentagesby mass: 0.95-1.10% carbon, 0.15-0.30% silicon, 0.40% or less manganese,0.020% or less phosphorus, 0.10% or less sulfur, 1.40-1.60% chromium,0.0015% or less oxygen, and remaining materials of iron and unavoidableimpurities.
 2. A finished wire for shot-peening materials prepared bythe steps of: wiredrawing the raw material for the shot-peeningmaterials of claim 1 to obtain a wire; and repeatedly annealing andcold-drawing the wire to obtain a finished wire, wherein an area ofcarbides with a particle size of 2 μm or less is 80% or more of a totalarea of the finished wire.
 3. The finished wire for shot-peeningmaterials of claim 2, wherein the annealing is performed at atemperature of 720° C. or lower.
 4. A method of manufacturingshot-peening materials comprising the steps of: cutting andplastic-forming the finished wire for the shot-peening materials ofclaim 2 to obtain raw shot-peening materials; and quenching andtempering the raw shot-peening materials.
 5. The method of manufacturingthe shot-peening materials of claim 4, wherein tempering parameters=T((21.3−5.8×[C])+log(t), where T denotes tempering temperature (K), ttempering time (hr), and [C] carbon content (%), are 6200-7300.
 6. Themethod of manufacturing shot-peening materials further comprising thestep of: plastic-forming the shot-peening materials that aremanufactured by the method of claim 4 or
 5. 7. A method of manufacturingshot-peening materials comprising the steps of: wiredrawing the rawmaterial for the shot-peening materials of claim 1 to obtain a wire;repeatedly annealing and cold-drawing the wire to obtain a finishedwire; cutting and plastic-forming the finished wire to obtain rawshot-peening materials; and quenching and tempering the raw shot-peeningmaterials.
 8. The method of manufacturing the shot-peening materials ofclaim 4, 5, or 7, wherein a temperature of the quenching is 820-850° C.9. The shot-peening materials manufactured by the method of claim 4 or7, wherein a matrix of a structure thereof is tempered martensite, andfine carbides precipitate therein, and wherein a ratio of an area ofcarbides to a total area is 70-95%.
 10. Shot-peening materialsmanufactured by the steps of: wiredrawing a raw material forshot-peening materials comprising, by percentages by mass, 0.95-1.10%carbon, 0.15-0.30% silicon, 0.40% or less manganese, 0.020% or lessphosphorus, 0.010% or less sulfur, 1.40-1.60% chromium, 0.0015% or lessoxygen, and remaining materials of iron and unavoidable impurities toobtain a wire; annealing and cold-drawing the wire to obtain a finishedwire; cutting and plastic-forming the finished wire to obtain rawshot-peening materials; and quenching and tempering the raw shot-peeningmaterials, wherein a matrix of a structure thereof is temperedmartensite, and fine carbides precipitate therein, and wherein a ratioof an area of carbides to a total area is 70-95%.
 11. The shot-peeningmaterials of claim 10, wherein a Vickers hardness thereof is 950 HV-1050HV.